Ok, but at least expert witnesses are constrained by the basic state of science in the field: They can certainly have a biased opinion but they can't go against established knowledge - and the other party can also interrogate them and try to show holes in their argumentation.

Whereas for jury members, the only people who could do that are other jury members, who would be just as clueless.

(I get that you don't want a jury with wildly different levels of domain knowledge. e.g. if you had one "expert" and the remainder being laymen, the expert could quickly dominate the entire jury - and there would be no one there to call out any bias from them)